1. Field of the Invention
The present invention relates to an electric power steering device of the type wherein an electric motor generates an assisting power to rotate a steering shaft of a motor vehicle.
2. Discussion of the Prior Art
Generally, in power steering devices, the ratio of a total steering power to a manual steering force or effort (hereafter referred to as "power assisting gain") is varied based upon various conditions such as vehicle speed, steering angle, steering angular velocity, road condition or the like so as to improve the steering feeling and the controllability. The total steering power which is required to operate the vehicle steering mechanism varies with variations in a resistance (i.e., steering load) that the steering shaft receives from a road surface, namely with the dependency of a steerable wheel restitution force upon the steering angle, as shown in FIGS. 7(a) and 7(b). This dependency varies with variations in the vehicle speed, the friction coefficient between tires and the road surface, the steering angular velocity, the direction in which the absolute value of the steering angle varies (i.e., further turn or return) or the like. The steering is easy if the required steering effort becomes smaller as the vehicle speeds decreases and does not increase as the steering wheel is further turned. However, the stability in high speed running is improved if the required steering effort is largely increased or made larger with increases in the steering angle.
By the way, known power steering devices are classified into hydraulic and electric types. Either type of these power steering devices is responsive to the vehicle speed, wherein a gain characteristic curve appropriate thereto is used to be amplified in accordance with a parameter of vehicle speed.
However, the hydraulic power steering devices have such a relation as shown in FIG. 6(b) between the steering load (which is proportional to the total assisting power in a condition that the steering angular velocity is constant) and the steering effort. More specifically, as the steering effort exceeds a predetermined value, the assisting power abruptly increases. This characteristic is advantageous for low speed running, because it provides for not only a proper stability of a steering wheel at the neutral position, but also a large assisting power with a small manual steering torque. However, when the gain of the characteristic shown in FIG. 6(b) is lowered for high speed running, the required steering effort does not increase at the same increasing rate as the the steering load. Thus, even when increases in the steering angle as well as in the steering angular velocity cause the steering load to increase, the required steering effort does not increase in proportion thereto. This provides the vehicle driver with an approximately constant steering feeling, thereby failing to obtain an enhanced controllability in high speed running.
On the other hand, in known electric power steering devices, the required steering effort is controlled to increase linearly with increases in the steering load, as shown in FIG. 6(a), and the gain (gradient) of the variation of the assisting power relative to the steering effort is changed with the vehicle speed. Therefore, the characteristic of the steering effort thus obtained is analogous to the characteristic according to which the steering load increases with increases in the steering angle and the steering angular velocity. This permits the steering wheel to become heavier as increases in the steering angle and the steering angular velocity so as to thereby provide for such a stability as required for high speed driving. However, according to the variation characteristics of the steering effort, the steering wheel becomes heavier with increases in the steering angle and the vehicle speed. Particularly, when the vehicle speed is zero, the steering load is extremely large to make the steering wheel too heavy to turn.